# Silver-oxide battery

A silver-oxide battery (IEC code: S) is a primary cell with a very high energy-to-weight ratio. Available either in small sizes as button cells, where the amount of silver used is minimal and not a significant contributor to the product cost.

Specific energy Silver oxide cells 130 Wh/kg[1] 500 Wh/L[1] High N/A Low High N/A

Silver-oxide primary batteries account for over 20% of all primary battery sales in Japan (67,000 out of 232,000 in September 2012).[2]

## Chemistry

A silver-oxide battery uses silver(I) oxide as the positive electrode (cathode), zinc as the negative electrode (anode), plus an alkaline electrolyte, usually sodium hydroxide (NaOH) or potassium hydroxide (KOH). The silver is reduced at the cathode from Ag(I) to Ag, and the zinc is oxidized from Zn to Zn(II).

The half-cell reaction at the negative plate:

${\displaystyle {\ce {Ag2O + 2H+ + 2e- -> 2Ag (v) + H2O}}}$ , ${\displaystyle (E^{\circ }=+1.17{\text{ V}})}$

The reaction in the electrolyte:

${\displaystyle {\ce {2H2O -> 2H+ + 2OH-}}}$ , ${\displaystyle (E^{\circ }=-0.83{\text{ V}})}$

The half-cell reaction at the positive plate:

${\displaystyle {\ce {{Zn}+2OH^{-}->{\overset {Zinc~hydroxide}{Zn(OH)2}}+2e-}}}$ , ${\displaystyle (E^{\circ }=+1.22{\text{ V}})}$

Overall reaction:

${\displaystyle {\ce {Zn + H2O + Ag2O -> Zn(OH)2 + 2Ag(v)}}}$ , ${\displaystyle (E^{\circ }=+1.56{\text{ V}})}$

Overall reaction (anhydrous form):

${\displaystyle {\ce {Zn + Ag2O ->[{\ce {KOH/NaOH}}] ZnO + 2Ag (v)}}}$

## Characteristics

Compared to other batteries, a silver-oxide battery has a higher open-circuit voltage than a mercury battery, and a flatter discharge curve than a standard alkaline battery[citation needed].

## Mercury content

Several sizes of button and coin cells, some of which are silver-oxide

Silver-oxide batteries become hazardous on the onset of leakage; this generally takes 5 years from the time they are put into use (which coincides with their normal shelf life). Until recently, all silver-oxide batteries contained up to 0.2% mercury. The mercury was incorporated into the zinc anode to inhibit corrosion in the alkaline environment. Sony started producing the first silver-oxide batteries without added mercury in 2004.[3]